As the nuclear utility industry matures, there is an ever-increasing need for additional storage space to safely contain spent nuclear fuel. One method that has been developed in recent years for storage of spent nuclear fuels is dry storage in horizontal storage modules, which are shielded bunkers in which containerized spent fuel is stored and monitored for definite periods of time. One conventional technique for horizontal modular dry storage of spent nuclear fuel rods is disclosed in U.S. Pat. No. 4,780,269 to Fischer et al.
A basic procedure for dry storage of spent nuclear fuel is to position a dry shielded canister into a shielded transfer cask. The canister and cask are filled with deionized water, which is then lowered into a pool containing the spent nuclear fuel. Spent fuel assemblies are then placed into the canister, and a shielded end plug is positioned to close the canister. The canister and cask are then removed from the pool, and the cask and canister are drained and dried. The exterior of the cask is decontaminated, followed by closure of the cask with a closure plate. The closed transportation cask is then lowered onto a transport trailer and secured by tie-downs.
The transport trailer carries the cask to the sight of the horizontal dry storage modules. The cask is opened and docked with an entry port of a dry storage module. The canister is then transferred from the cask into the module, such as by passing a ram through the dry storage module from an end opposite the entry port, through the entry port and into the opened cask. The canister can then be grasped and pulled into the dry storage module, after which both the entry port and access port are sealed.
A critical aspect of this process is the safe containment and transfer of the spent nuclear fuel within the canister from the original pool storage to the final dry horizontal storage site. The transport cask must be constructed with adequate structural strength and shielding to both physically protect the dry shielded canister within, and to provide biological shielding to minimize personnel radiation dosages during canister transfer and transport operations.
During the canister transfer and transport process, the cask must be able to withstand any foreseeable impact, such as could occur by accidental dropping of the cask from the transport trailer or exposure to tornadoes or other natural disasters. In the United States, federal regulations setting forth requirements that transport casks must meet are found in 10 C.F.R. 72, including subpart G, as well as 10 C.F.R. 71 and 10 C.F.R. 50. In particular, the cask must be able to withstand impacts due to a drop of 30' onto an essentially, unyielding flat horizontal surface, without structural failure. Even if structurally damaged, no leakage of the contents from the cask is permitted.
It is thus important to design casks with high structural integrity. At the same time, it is desirable to maximize the quantity of spent fuel that can be transported within the cask at any given time, and to minimize the cost of constructing the cask. While strength considerations typically warrant constructing the cask from thicker sections of metal and other materials, this requirement may reduce the quantity of spent fuel that can be transported within the cask. External dimensions of the cask are limited by constraints such as the total weight of the loaded cask, and clearances required to transport the casks through tunnels, under bridges and overpasses, and the like.
Currently, conventional casks are often constructed from a polished austentitic stainless steel, such as 304 stainless steel, for corrosion prevention. However, such stainless steel is limited in strength and may fail under high stresses. To combat this potential, conventional casks are constructed from thick metal sections, and must be reinforced with gusset plates and other reinforcing members. Additionally, locations on the casks that are subjected to force during transport must be reinforced with additional metal plates welded to the cask structure.
For example, conventional casks are outfitted with cylindrical trunnions welded or bolted directly to a cylindrical structural shell of the cask at diametrically opposed locations. These trunnions are grasped by hooks, and serve as pivot points while lieting the cask during the transportation process. Because of the stresses transferred to the cask structure from the trunnions during use, the shell is typically reinforced in the area surrounding the trunnions by welding additional plates of metal.
The trunnions themselves are conventionally permanently secured to the structural shell of casks by welding or bolting directly to the shell. In the case of welding, the welded joint is subjected to substantial stress during hoisting of the cask. In the case of bolting the trunnions in place, the bolts are subjected to extreme shear and tensile loads during hoisting of the cask. Again, the trunnions must be heavily reinforced to withstand such loads, increasing the weight and overall dimensions of the cask, and thus decreasing the spent fuel containment capacity and increasing the cost of manufacture.
When sealed joints, such as elastomeric (e.g., O-ring) seals or metal seals are utilized, the base metal used to form the structural shell is conventionally machined to form the sealing surfaces. Thus, for example, when 304 stainless steel is used to construct the shell, annular surfaces on the shell are machined and polished to form sealing surfaces. While functioning adequately in most situations, extreme impact to the seal area, such as by accidental dropping of the cask at an oblique angle whereby force is concentrated on the seal area, may result in permanent deformation of the metal seal surface, and subsequent leakage potential.